The class I major histocompatibility antigens in the hamster are unique among species that have been closely examined because they appear to be nonpolymorphic. The organization of the class I gene complex is now under investigation to attempt to determine the reason for this unusual behavior. A number of class I genes have been cloned from a genomic library and mapped. The identity of the expressed cell-surface antigens will be determined by examining cDNA clones. The functional significance of changes in class I expression is being investigated in hamster and mouse cells transformed with oncogenic and nononcogenic adenoviruses. The oncogenic adenovirus 12 appears to rely on elimination of class I antigen expression in the transformed cell to escape host detection. The nononcogenic adenoviruses 2 and 5 have no such effect. Ad2 and Ad5 transformants expressing low levels of class I antigens have been found but they are still nontumorigenic. In addition, adult adaptation of an Ad2 transformant led to high tumorigenicity without a change in class I expression. Thus, class I antigen expression is irrelevant to tumorigenicity by the nononcogenic adenoviruses. Hamster female protein has been examined with respect to its disulfide bond arangement. Each subunit has two cysteine involved in an intramolecular disulfide bond. The third cysteine in each subunit is the most carboxy-terminal one and is present in the reduced state.